1. Field of the Invention
The invention relates to a structure of a trench capacitor and method for manufacturing the same, and more particularly, to a method of manufacturing a trench capacitor with a protective layer covering on the collar oxide layer. In this way, the reliability of a failure analysis can be improved.
2. Description of the Prior Art
As electrical products tend towards increasing miniaturization, DRAM devices need to have a high integration and density. Trench capacitor DRAM devices are popularly used as high density DRAMs formed in a deep trench capacitor of the semiconductor substrate to effectively decrease the size of the memory unit and efficiently utilize the space of the chip.
Please refer to FIG. 1 to FIG. 5, which are schematic cross-sectional views showing the fabrication of the trench capacitor according to the prior art.
As shown in FIG. 1, a substrate 10 having a STI 12, a pad silicon nitride 14, a buffering silicon nitride 16 and a plasma enhanced oxide (PEOX) layer 18 is provided. An etching process is performed to form a trench 20. As shown in FIG. 2, an oxide layer 22 is deposited on the surface of the trench 20 and the PEOX layer 18. As shown in FIG. 3, a trench 24 is formed by taking the oxide layer 22 as a mask and the trench 24 extends from the bottom of the trench 20 into the substrate 10. The remaining oxide layer 22 serves as a collar oxide layer 32. As shown in FIG. 4, a bottom electrode 26 is formed on the sidewall of the trench 20, 24. As shown in FIG. 5, a capacitor dielectric layer 28 is formed to cover the bottom electrode 26. Then, a conductive material 30 serving as a top electrode fills up the trench 20, 24. Finally, the pad silicon nitride 14, the buffering silicon nitride 16 and the PEOX layer 18 are removed.
According to the conventional method, the collar oxide layer 32 is made by the following steps: first, dry etching the oxide layer 22 to form a spacer-like structure on the sidewall of the trench 20; and etching the substrate 10 by taking the spacer-like structure as a mask. The trench 24 is formed in this way. In the above steps, most of the oxide layer 22 is removed, and the remaining oxide layer 22 is taken as the collar oxide layer 32. In other words, part of the oxide layer 22 serves as a mask during the trench 24 formation, and part of the oxide layer 22 serves as the collar oxide layer 32. Therefore, to assure the collar oxide layer 32 has a sufficient thickness, a thicker thickness of the oxide layer 22 is formed before the etching process in the conventional process. If the etching time is not well-controlled, for example, a slight increase in the etching time will lead to over-etching of the oxide layer 22 and the collar oxide layer 32 will be too thin to provide a good insulating function. Therefore, the trench capacitor will have a current leakage problem. On the other hand, if the etching time is not long enough, the collar oxide layer 32 will be too thick and the capacitance of the trench capacitor will decrease. Furthermore, the quality of the deposited electrode will also be deteriorated.
Moreover, during a failure analysis process, the conventional detecting machine has a problem in determining the thickness of the collar oxide layer 32, which leads to an inaccurate failure analysis.
Therefore, it is important to provide a fabricating method to form a collar oxide with a suitable thickness, and to improve accuracy of the failure analysis process.